It has been shown that dextran is biosynthesized from the reducing end by an insertion mechanism. It was further shown that during the polymerization the enzyme forms a glucosyl and a dextranosyl complex. In the present experiments dextransucrase was charged with sucrose. The sucrose was removed and the charged enzyme was reacted with (14C) reducing-end labeled maltose which acted as an acceptor substrate . The reaction resulted in the release of two types of labeled products, a high molecular weight dextran and a mixture of oligosaccharides. The position of the label in the released dextran and the oligosaccharides was found to be at the reducing end by the formation of labeled sorbitol after NaBH4 reduction and hydrolysis. This indicated that the mechanism of acceptor reactions is the nucleophilic displacement at the reducing end of glucosyl and dextranosyl intermediates from the enzyme. Similarly when a charged and 14C labeled dextransucrase was incubated with a low molecular weight dextran, in the absence of sucrose, the label was released as a high molecular weight dextran. Acetolysis of this released dextran gave 7.3 percent of the label in nigerose. Reduction of the nigerose, followed by hydrolysis, gave only labeled glucose indicating that the nigerose was exclusively labeled in the nonreducing glucose moiety. This result indicated that the formation of alpha-1 yield 3 branch linkages in dextran biosynthesis occurs by a nucleophilic displacement on C1 of the reducing end of a dextranosylenzyme complex by a C3-OH on an acceptor dextran. These acceptor reactions serve to terminate dextran polymerization by displacement of the growing chain from the enzyme and to form alpha-1 yield 3 branch linkages when the displacement is made by another dextran chain. BIBLIOGRAPHIC REFERENCES: Hajime Taniguchi and John F. Robyt, "Branching Mechanism in Dextran Biosynthesis by Dextransucrase from Leuconostoc mesenteroides B-512," Federation Proceedings, 34:575 (1975). John F. Robyt and Hajime Taniguchi, "The Mechanism of Dextransucrase Action: Biosynthesis of Branch Linkages by Acceptor Reactions with Dextran," Arch. Biochem. Biophys. in press (1976).